A diesel engine is a type of internal combustion engine that uses the heat generated through compression of fuel and oxygen to initiate the ignition of that fuel that has been injected into the combustion chamber of the engine. In the combustion chamber, the fuel is combusted in the presence of oxygen to produce high-temperature and high-pressure combustion products which apply a force to one or more components of the engine, thereby providing mechanical energy. The exhaust gas emitted from a diesel engine may contain various gaseous emissions such as nitrogen oxides (NOx) which may contain nitrogen dioxide (NO2). When released into the atmosphere, NO2 may develop as a yellow-brown haze over cities, and is one of the components of smog.
In an effort to decrease the amount of NOx released into the atmosphere, increasingly stringent emission standards require limiting the amount of NOx emissions. Specifically, in the United States, regulations may limit NOx emssions to 0.4 g/kW·h for off-road diesel engines having brake horsepowers between 175-750. One type of diesel engine exhaust treatment technology that has been used to reduce NOx emissions from diesel engines is a selective catalytic reduction (SCR) aftertreatment system. In operation, a reductant, such as an aqueous urea solution, also referred to as diesel exhaust fuel (DEF), is injected into the exhaust gases upstream of an SCR catalyst. The urea may decompose to ammonia and, in the presence of the SCR catalyst, the ammonia may serve as a reducing agent to reduce the NOx in the exhaust steam to nitrogen and water which are then expelled through the tailpipe. In this way, SCR aftertreatment systems may reduce NOx emissions by 90 percent or more.
In higher horsepower diesel engines with higher exhaust gas volumes, multiple DEF injectors and/or multiple SCR aftertreatment modules may be placed in the exhaust stream to effectively meet emission standards. For example, U.S. Patent Application Publication number 2011/0023466 describes the use of multiple nozzle-injectors for delivering DEF to an exhaust gas of a diesel engine upstream of an SCR catalyst.
While effective, current DEF dosing systems may suffer from a number of drawbacks. For example, the pump used to deliver the DEF to the injector(s) may have a pumping capability which exceeds the dosing capability of the injector(s), such that the entire pumping capability of the pump is not fully used. In addition, accurate control and prediction of the amount of DEF injected into the exhaust stream may be complicated by the tendency of DEF to form deposits in the exhaust line which may not reach the SCR catalyst for NOx reduction. Furthermore, it may be difficult to meet the energy requirements for heating systems used to thaw DEF supply lines exposed to low temperatures within government-specified timeframes, particularly when using multiple DEF supply lines and/or longer DEF supply lines.
Clearly, there is a need for improved DEF dosing system designs for SCR aftertreatment modules in diesel engines.